Subject: Re: [PATCH] Multicast packet reassembly can fail

From: Eric Dumazet <[email-masked]>
Date: Wed, 28 Oct 2009 11:18:24 +0100

Steve Chen a écrit :
> Multicast packet reassembly can fail
> 
> When multicast connections with multiple fragments are received by the same
> node from more than one Ethernet ports, race condition between fragments
> from each Ethernet port can cause fragment reassembly to fail leading to
> packet drop.  This is because packets from each Ethernet port appears identical
> to the the code that reassembles the Ethernet packet.
> 
> The solution is evaluate the Ethernet interface number in addition to all other
> parameters so that every packet can be uniquely identified.  The existing
> iif field in struct ipq is now used to generate the hash key, and iif is also
> used for comparison in case of hash collision.
> 
> Please note that q->saddr ^ (q->iif << 5) is now being passed into
> ipqhashfn to generate the hash key.  This is borrowed from the routing
> code.
> 
> Signed-off-by: Steve Chen <[email-masked]>
> Signed-off-by: Mark Huth <[email-masked]>
> 

This makes no sense to me, but I need to check the code.

How interface could matter in IP defragmentation ?
And why multicast is part of the equation ?

If defrag fails, this must be for other reason,
and probably needs another fix.

Check line 219 of net/ipv4/inet_fragment.c

#ifdef CONFIG_SMP
        /* With SMP race we have to recheck hash table, because
         * such entry could be created on other cpu, while we
         * promoted read lock to write lock.
         */
        hlist_for_each_entry(qp, n, &f->hash[hash], list) {
                if (qp->net == nf && f->match(qp, arg)) {
                        atomic_inc(&qp->refcnt);
                        write_unlock(&f->lock);
                        qp_in->last_in |= INET_FRAG_COMPLETE;   <<< HERE >>>
                        inet_frag_put(qp_in, f);
                        return qp;
                }
        }
#endif

I really wonder why we set INET_FRAG_COMPLETE here